United States patent specification No. 3,858,040, in the name of the present Applicant or an assignee of the present Applicant, defines a cut-off profile which enables the above standard to be complied with. This cut-off line is defined by two horizontal half planes P1 and P2 which are offset from each other in height, with the right hand half plane being offset upwardly for use when driving on the right, and by an oblique junction zone in the vicinity of the axis defining the direction of travel.
The same United States patent specification also discloses a headlamp having a reflector of the parabolic type which enables this cut-off line to be generated, with a filament which is provided with an occulting shield or mask that enables a light beam to be produced which is chopped in a horizontal plane.
This headlamp also has a cover glass which is provided with prisms for raising the images of the filament lying on one side, so as to define the raised cut-off half plane P2. It will be understood that, because the occulting shield cuts out about one half of the light emitted, this headlamp only produces a rather weak light output.
It is also known, from French patent specification No. FR 2 583 189A, to provide a headlamp which is capable of producing, with a filament which has no occulting shield, and by virtue of a specific design of the reflector, the same type of cut-off line. In one embodiment of that headlamp, it is necessary to arrange prisms on the cover glass so as to obtain the two offset half cut-off lines. In another embodiment of the same headlamp, this offset is obtained by means of an angular offset of certain regions of the reflector with respect to others, in such a way that the reflector has undesirable interruptions which give rise to optical anomalies.
Finally, French patent specification No. FR 2 599 120A, which is again in the name of the Applicant or an assignee of the Applicant, discloses a headlamp which is capable, again with the aid of a reflective surface which has no discontinuity, of producing a light beam which has the same type of cut-off line, and in which the pool of concentration of light in the beam is offset laterally with respect to the axis of the filament and reflector.
However, in these various known types of headlamp, the light beam which is produced by the bare reflector remains relatively narrow, and major adjustment is required in the region of the cover glass.